1. Field of the Invention
The present invention relates to a controller for a heating cable. In particular, but not exclusively, the present invention relates to a controller arranged to control the flow of current to a heating cable in order to control the temperature of the heating cable.
2. Description of Related Art
Conventionally, a heating cable comprises a long assembly having at least one conductor extending along the length of the cable, and an insulation layer surrounding at least one conductor. A heating cable may commonly form part of a heating blanket. When part of a heating blanket the heating cable is typically arranged in coils or in a sinuous pattern throughout the blanket sandwiched between layers of material. An electrical current is passed through at least one conductor. The electrical current causes the conductor to heat up transferring the heat to the blanket and indirectly to the user, or the object to be heated.
Safety is a major issue for heating blankets, particularly with heating blankets that are used to warm for example bedding. The primary safety issue is that of over heating. Serious injury and some times death can occur, for example as a result of bedding catching fire due to the heating blanket over heating.
A secondary, but nevertheless significant issue is that of exposure of the user to an increased electromagnetic field (EMF). This arises as a result of a user being in close proximity to a conductor carrying an alternating current. It is becoming increasingly important to reduce the EMF emitted by consumer products both for health and product standard reasons. It is desirable that any controller arranged to reduce the risk of a heating cable over heating is also suitable for operation with a heating cable with a reduced EMF. To reduce the EMF emitted a dual heating element construction can be used which comprises two conductors. The two conductors are arranged such that current flows in opposite directions along the conductors. The effect of this is that the EMF emitted by one conductor is substantially or completely cancelled out by the EMF emitted by the other conductor. To achieve this opposite flow of current along the cable, each conductor is connected to a separate pole of a power supply at a first end of the cable, and the two conductors are joined together at the other end of the cable.
Many modern heating cables do not require separate thermostats in order to control their temperature. Such heating cables can be conveniently split into two categories. Firstly, there are those heating cables having a separation layer disposed between two conductors, the electrical resistance of the separation layer having a positive temperature coefficient (PTC). As the temperature of the cable increases, the resistance of the separation layer increases. Such a heating cable is self limiting as an increase in temperature leads to a decrease in current flow, and therefore reduced heating. Secondly, there are those heating cables that provide a feedback signal to a controller for monitoring the temperature of the cable. Embodiments of the present invention relate primarily to a controller for a heating cable falling into the second category.
Appliances incorporating heating cables are not always static in use. As a result folds can occur in the appliance and localised heating can arise due to an uneven distribution of the heating cable. For under floor heating, if part of the floor is covered an area of localised over heating can be created as it is more difficult for the generated heat to escape. It is known to have a heating cable wherein at least one conductor has a PTC characteristic such that as the resistance of the conductor increases with a rise in temperature. This resistance increase is detectable and the current supplied to the cable can be reduced accordingly. It is very difficult to detect a localised area of over heating with such a cable as a localised change in resistance will have a negligible effect on the overall measured resistance. For a heating cable with separate thermostat control it is unlikely that the thermostat would detect a localised area of heating unless the affected area is proximal to the thermostat.
Another known technique is to have a dual conductor construction with a fusible separation layer disposed between the two conductors. In the event of localised over heating the separation layer melts shorting out the cables. This leads to a large current flow, which is arranged to blow a fuse cutting off the power supply. While this solution works effectively, once the separation layer has melted the appliance is unusable. This is particularly undesirable when the cable is used in a hard to maintain location, such as is the case for under floor heating.
An early more sophisticated attempt to address the over heating issue is described in U.S. Pat. No. 3,375,477. This document describes a heating cable made up of a first conductor through which a heating current flows, and a second conductor, which extends alongside but is separated from the first conductor by a separation layer. The electrical resistance of the separation layer has a negative temperature coefficient (NTC) such that the resistance of the separation layer reduces with increasing temperature. For an area of localised heating the resistance of the separation layer in that area will reduce. The separation layer can be thought of as many parallel resistors along the length of the heating cable. Therefore a reduction in resistance in one area has a significant effect on the average resistance of the whole separation layer. Consequently the leakage current across the separation layer from the first conductor to the second conductor increases significantly. This leakage current can be detected and used to interrupt the supply of power to the first conductor in the event that the leaking current exceeds a predetermined threshold.
Generally a heating cable in accordance with U.S. Pat. No. 3,375,477 is supplied with a controller, which includes a circuit designed to cut off the supply of power if the current drawn by the heating element exceeds a predetermined threshold. Thus the overall assembly can be considered as a two-safety feature system. The NTC separation layer is designed so that the separation layer is not destroyed in the event of over heating and therefore the heating cable is not designed to be rendered permanently inoperable as a result of being subjected to an excess temperature on one occasion.
In a development of the basic concept of relying upon an NTC separation layer to detect over heating, it is known to use a separation layer which is both NTC and fusible. Such an arrangement is described in U.S. Pat. No. 6,310,332. In the described arrangement, normal power supply control is achieved by monitoring the NTC characteristics of the separation layer. If however abnormally high temperatures are reached at any point along the length of the heating cable the separation layer will melt, enabling the two conductors of the coaxial assembly to come into direct contact, thereby causing a short circuit between the two conductors. Such a short circuit can be detected and used to cut off the power supply. Once this has occurred the product is of course effectively destroyed as the product cannot be returned to a normal operative condition.
U.S. Pat. No. 6,310,332 describes two embodiments, that is the embodiment of FIG. 1 and the “more functional” embodiment of FIGS. 2 and 3 of that patent. In the embodiment of FIGS. 2 and 3, one conductor carries the heating current whereas the other is used for sensing purposes. The electrical resistance of the sensing conductor may also have a positive temperature coefficient (PTC) to provide an additional means for monitoring temperature along the length of the cable. With that arrangement, however, the EMF issue is not addressed as the sensing cable does not carry the heating current.
In contrast in the embodiment of FIG. 1 of U.S. Pat. No. 6,310,332, two heating wires are connected in series by a diode, with heating current passing through each of the heating wires. The diode means that only the positive half cycles of the AC supply can pass through the cable. During the negative half cycles the controller measures the leakage current through the separation layer, which bypasses the diode. Leakage of current through the separation layer is detected by the appearance of a current flowing in the opposite direction to the normal direction of current flow through the diode. This arrangement addresses the EMF issue as current in the two heating wires flows in opposite directions along the cable. However, only half the available power is used to heat the heating cable due to the presence of the diode. As such the resistance of the heating cable must be half that possible when the full AC cycle is used. This can create manufacturing problems, as cables become more inflexible as the resistance lowers.
It is desirable that a controller can be used in a range of products incorporating heating cables of different lengths.
It is an aim of embodiments of the present invention to obviate or mitigate one or more of the problems of the prior art, whether identified herein or elsewhere.
It is an aim of embodiments of the present invention to provide a new type of controller for a heating cable capable of detecting a change in temperature of a connected heating cable and adjusting the power supplied to the heating cable accordingly.
In preferred embodiments of the present invention the controller is arranged to prevent permanent damage being caused to either itself or the heating cable in the event of the cable over heating.
In preferred embodiments of the present invention the controller is arranged to measure two parameters of the heating cable to provide two independent forms of over heating protection. The two parameters are suitable for detecting over heating in a localised section of the heating cable and an average over heating along the whole length of the cable respectively.
It is an aim of embodiments of the present invention to provide a controller for a heating cable suitable for use with a cable that inherently has a good EMF performance.
It is a further aim of embodiments of the present invention to provide a controller for a heating cable capable of being used with heating cables of different lengths.